ROOF FAILURE STEEL BUILDING CBC

[bdruehl]

i do not have the resources or time or (brain power maybe) to analyze the entire steel building done by third party 20 years ago. So.....

the utmost top roof members (8" deep z channels) failed under a heavy snow load (which the building was not designed for...apparently didnt have to be because of county lines, etc..etc..). Designing the fix, we are replacing the failed z channels with much bigger members (huge conservative 50# snow load), basically adding around 9000 lbs of dead load to the structure. Needless to say, the big girts holding up these new beams need to posted midspan under this load...

Im in SDC D... i would like to use the post as a cantilever (omf?) to resist the additional seismic loading from the additional weight of the new beams ONLY ... seems ASCE7 allows OMF 12.2.5.7... could flag pole design be used?? horrible R value yes... I cant make head nor tails of the code... any input is appreciated... and yes, i realize that by tributary rule, the posts should take half of ALL the building lateral, but for sake of this design... lets go with it!? (still attempting to read the old microfiched steel building plans to decipher the loadings... maybe wind still controls! doubt it....cant read it anyways...)

 

[JAE]

bdruehl - don't have time to fully respond to your post but "tributary rule" doesn't apply to lateral seismic forces.

Seismic forces should be distributed to the various elements based upon relative rigidity of the different lateral force resisting elements. Also, center of mass influences where the demand is applied which can affect how much each element resists.

So a rigid bent in your metal building might take more lateral than the center cantilevered post simply due to higher lateral stiffness.

 

[hokie66]

So you are reinforcing the girders (not girts) by providing new columns at somewhere in the span. Assuming this is a portal frame, why not provide moment connections from the new columns to existing rafters to provide additional lateral strength as well as vertical?

 

[bdruehl]

yes indeed JAE! holy moly, thanks for the reminder about the the relative stiffness concept, and all your other posts that i have seen everywhere!! too caught up in wood shearwall land<--- snap out-of it

... well, the same problem will exist, i need to analyze the entire building if i go down that avenue,,, basically, it seems that designing the posts to only take the increased lateral from the additional weight of the new roof members is at best,kinda dubious??

hookie! and yes, the girDers need some help now, which is where the new columns come in midspan, (yes is a portal frame) which is what i was talking about for also adding some lateral resistance! but, other than an assumptive statement saying "building good for siesmic after increase dl because new columns (w moment connections) designed for the extra lateral from seismic"... there should be some proof in the pudding... and then there are the code issues for Moment Frames in siesmic D? etc. etc... any genius way of a mathematical solution to what the columns would need to resist. other than a copout = just the extra deadload? thanks for the input...

cant remember if ive ever seen a steel building calc by hand, steel bulding design is notsomething i already have in my "repertoire"... could you tell?

 

[bdruehl]

checking for wind vs seismic control with increased roof dead load... i suppose i have to use 3 for R??